1. Field of the Invention
The present invention relates to a measuring device, and particularly to a measuring device for detecting magneto-optical effects such as a magnetic Kerr effect and a Faraday effect.
2. Description of the Related Art
In order to measure a magnetization state of a magnetic substance, a method of irradiating light to the magnetic substance and detecting change in polarization state and reflectivity of the reflected light due to a magnetic Kerr effect is widely being used. For example, with a device called a Kerr effect measuring device, a Kerr effect magnetic domain observing device, and the like, M-H characteristics can be measured and magnetic domains can be observed by a camera, and a linearly polarized light source in which output of a lamp light source is passed through a polarizer or a laser light source is used. Further, there is a device called a micro Kerr measuring device, a scanning-type Kerr effect microscope, and the like for observing a magnetization state of a fine region, in which magnetization at a laser focusing spot can be measured by using the laser light source and focusing light by an object lens to detect polarization of the reflected light by a sample, and a magnetized distribution image can be obtained by conducting the magnetization measurement while scanning the sample or the position of the laser spot.
Furthermore, using a pulsed light source in these devices, a method of conducting stroboscopic image-observation of magnetized distribution which changes at high speed (for example, M. H. Kryder and F. B. Humphrey, “Dynamic Kerr Observations of High-Speed Flux Reversal and Relaxation Processes in Permalloy Thin Films”, Journal of Applied Physics Volume 40, Number 6, pp. 2469–2474) and a method of measuring high-speed magnetization responses at a laser focusing position (for example, M. R. Freeman and J. F. Smyth “Picosecond time-resolved magnetization dynamics of thin-film heads” J. Appl. Phys. 79(8), 15 Apr. 1996) are also used.
Moreover, the magnetic Kerr effect is also used for reading magnetic recording data of a magnetic medium in a magneto-optical recording device.
When collimated light having linear polarization is incident, the magnetic Kerr effect is classified into three kinds of a polar Kerr effect, a longitudinal Kerr effect, and a transverse Kerr effect according to relationship between a magnetized direction of the magnetic substance and a plane of incidence of the light.
The polar Kerr effect occurs when the magnetized direction of the magnetic substance is perpendicular to a surface of the magnetic substance, and rotation and ellipticity of polarization of a reflected light are caused according to magnetization value when linearly polarized light of P-polarization or S-polarization is incident.
The longitudinal Kerr effect occurs when the magnetized direction of the magnetic substance is parallel to the surface of the magnetic substance and the plane of incidence, and rotation and ellipticity of polarization of the reflected light are caused according to magnetization value when the linearly polarized light of P-polarization or S-polarization is incident.
The transverse Kerr effect occurs when the magnetized direction of the magnetic substance is parallel to the surface of the magnetic substance and perpendicular to the light incident plane, and it is a phenomenon that reflectivity changes according to magnetization value, and rotation and ellipticity of polarization are not caused when the linearly polarized light of P-polarization is incident while change according to magnetization value is not caused when the linearly polarized light of S-polarization is incident.
In the Kerr effect measuring device for measuring magnetic characteristics of a magnetic film, which does not require high spatial resolution, these three kinds of Kerr effects can be separately measured by irradiating the collimated light from the laser light source to the magnetic film as it is in the P-polarization state or the S-polarization state without being focused, and observing change in polarization rotation, ellipticity of polarization, and light amount of the reflected light.
On the other hand, in the micro Kerr measuring device for observing the magnetization state of the fine region with high spatial resolution, the light from the lamp light source or the laser light source is focused by a focusing lens and irradiated to the magnetic substance, and the reflected light is focused by the same focusing lens to conduct observation. In this case, the focused light to be irradiated to the magnetic substance includes incident planes having continuously different azimuth angles and, further, includes beam components having continuously different incident angles for the light having the respective incident planes. Furthermore, considering the focused light separated by the respective incident planes, even if the light from the light source is linearly polarized light having a fixed polarization azimuth, a P-polarization component and an S-polarization component exist at different ratios according to the azimuth angles of the incident planes. Therefore, considering the focused light flux separately by each of the different incident planes, only the polar Kerr effect is caused by a perpendicular magnetization vector component from the definition while there exists an incident plane where both of the longitudinal Kerr effect and the transverse Kerr effect exist together caused by an in-plane magnetization vector component irrespective of the relationship between its direction and the polarization azimuth of the light of the light source. The transverse Kerr effect does not cause polarization rotation of the reflected light of P-polarization and S-polarization but causes effective rotation of the polarization azimuth of polarized light having a general azimuth because of change in reflectivity of the P-polarization component thereof. Accordingly, even if a polarization detection system for detecting polarization rotation of the reflected light is used, the transverse Kerr effect generally exerts influence on each of the incident planes when a focusing and irradiation optical system is used. As stated above, when the light is focused by the object lens and irradiated to the magnetic substance, the polar Kerr effect, the longitudinal Kerr effect, and the transverse Kerr effect are complicatedly mixed, and special conditions become necessary for measuring only one component among the perpendicular magnetization vector component and two orthogonal in-plane magnetization vector components completely separately.
In order to measure the magnetic component perpendicular to the surface of the magnetic substance to be measured with high spatial resolution, the laser light outputted as linearly polarized light is made incident in an axially symmetric manner with respect to the optical axis center of a pupil of the object lens without deviation, and focused and irradiated to the surface of the magnetic substance, and its reflected light is focused by the same object lens while keeping the axial symmetry nature so as to detect change of polarization state thereof uniformly in the whole of the light flux. In this case, the longitudinal Kerr effect and the transverse Kerr effect generated by the in-plane magnetization vector components cancel each other as the whole of the light having the different incident angles and planes, which is focused in the axially symmetric manner, and the polar Kerr effect, namely, the perpendicular magnetization vector component remains.
In measuring the in-plane magnetization vector components, a method of deviating a position of the incident light on the object lens pupil is general, in which, for example, the light is made incident on a half of the object lens pupil with its light incident angle being deviated from the perpendicular direction to the surface of the magnetic substance, and the reflected light is taken from the other half of the object lens pupil to detect the polarization state so that measurement is conducted using the longitudinal Kerr effect and the transverse Kerr effect (for example, Semiconductor and Material Department Meeting of Electronic Communication Society, 1983, p. 48, “Magnetized distribution measurement of micropermalloy patterns”, Nonaka et al., and Japanese Patent Application Laid-open No. Hei 5-215828 “Magnetic Domain Structure Analyzing Device”.
Moreover, described in Japanese Patent Application Laid-Open No. Hei 6-236586, “Magneto-Optical Recording Medium, Magneto-optical Recording And Reproducing Method Using The Same, And Magneto-Optical Recording And Reproducing Device” is a method of reading magnetized recording information in two directions in a flat plane using two kinds of light sources having different polarization azimuths.
In addition, also used is a method of separately detecting the perpendicular magnetization vector component and the in-plane magnetization vector components, in which the laser light is made incident on the object lens pupil without being deviated from the optical axis center when the light is incident on the object lens and the reflected light is focused by the object lens, similarly to the measurement of the polar Kerr effect, and components of the focused incident/reflected light by the object lens are separately detected using a two-divided or four-divided photo-detector and a detection signal of each divided part of the photo-detector is electrically added and subtracted so that the each component of magnetization vector is detected (for example, W. W. Clegg, N. A. E. Heyes, E. W. Hill, and C. D. Wright, “Development of a scanning laser microscope for magneto-optic studies of thin magnetic films”, J. Magn. Magn. Mat., vol. 95, pp. 49–57, 1991, R. J. M. Veerdonk, Ganping Ju et al., “Real-time observation of sub-nanosecond magnetic switching in perpendicular multilayers” Journal of Magnetism and Magnetic Materials 235 (2001) p. 138–142, Journal of Magnetic Society of Japan, Vol. 23, No. 12, 1999, “Techniques for Analysis of Magnetic Recording Heads and Magnetoresistive Heads” Hiroyuki Ohmori.
In the method of deviating intensity distribution of the light incident on the object lens pupil, for example, the method in which the light is made incident on the half of the pupil with its light incident angle being deviated from the perpendicular direction to the surface of the magnetic substance, and the reflected light is taken from the other half of the object lens pupil to detect the polarization state so that the in-plane magnetization vector components are measured using the longitudinal Kerr effect and the transverse Kerr effect, there is a disadvantage that the polar Kerr effect by the perpendicular magnetization vector component is also detected at the same time. Further, it is easy to make a light beam having the diameter approximately half of that of the pupil incident on the half of the pupil in order to efficiently use the light from the light source but, in this case, spatial resolution is degraded because actually-used effective numerical aperture NA among the NA of the object lens becomes substantially small. If the light beam from the light source having the diameter approximately the same or larger than the diameter of the object lens pupil is made incident on the object lens with a half thereof being shielded, the effective NA can be made somewhat larger than in the aforesaid method, but there still exists a problem that spatial resolution is degraded and light usage efficiency also considerably lowers.
Furthermore, described in Japanese Patent Application Laid-open No. Hei 6-236586 is the method of reading magnetized recording information in two directions in the flat plane using the two kinds of light sources having different polarization azimuths, and a method for realizing it has the structure in which two heads as light sources are provided or two light sources having different wave-length are used and the light sources for outputting two polarized light are independently placed to separate light into two polarization optical paths and thereafter detect two polarization rotation by two independent polarization rotation detectors. In this case, since the two light sources operate incoherently and independently, an obtained light intensity focusing spot becomes a spot resulting from overlaying light intensity focusing spots which are independently formed by the respective two light sources, which reduces the effective NA similarly to the above description and lowers focusing performance. Furthermore, a realizing method for making detectivity to the perpendicular magnetization vector component zero is not described. Moreover, a method for realizing measurement of the perpendicular magnetization vector component is not described nor a method for realizing measurement of the perpendicular magnetization vector component separately from the in-plane magnetization vector components is not described. In addition, since the two independent light sources and polarization detectors are used, the structure is complicated and not easy to adjust.
Further, in the method of separately detecting the perpendicular magnetization vector component and the in-plane magnetization vector components using the two-divided or four-divided photo-detector, an amplifier such as an I/V conversion amplifier for amplifying an output current signal of each element of the divided photo-detector, or the like becomes necessary, and each output of the amplifier is needed to be added and subtracted, which degrades an S/N because amplifier noise is also added at this time. For example, when two outputs of an amplifier having the same noise power are added or subtracted, a noise voltage becomes √{square root over ( )}2 times as compared with a case of one I/V conversion amplifier even though noise generated by an add-subtract circuit (here, gain of the add-subtract circuit is supposed to be 1 for convenience of the explanation) is neglected and, when four outputs are added or subtracted, the noise voltage becomes twice. Since the magnetic Kerr effect is feeble, a fine signal buried in circuit noise or other noise needs to be extracted by signal processing, and a problem that measurement accuracy directly lowers arises if noise is thus increased.
Furthermore, the light incident position on the two-divided or four-divided photo-detector needs to be precisely adjusted so that a divisional detection ratio of the light becomes a predetermined ratio, that is, the light is equally distributed to each divided part in general, and this adjustment becomes difficult if an element having a small light receiving area is used. Moreover, in the case of a differential polarization detecting optical system in which two of the divided photo-detectors are used and two orthogonal polarization components are detected to obtain difference therebetween, light amount noise is canceled and a high S/N is easily obtained but, in this case, light incident positions on the two divided detectors have to be adjusted at the same time, which becomes more difficult.
Meanwhile, in the case of a photo-detector having a large light receiving area, since junction capacitance of the detector becomes large and high-speed response is not easily obtained as well as it is difficult to realize an I/V amplifier capable of giving the high-speed response while having large input capacitance, a problem that a high-speed light receiving and detection system cannot be formed arises. In addition, it is also difficult to realize an add-subtract circuit which accurately operates at a high frequency.
Further, in the method of using the two-divided or four-divided photo-detector, the center of the reflected light flux reflected by one point of the magnetic substance needs to be aligned with the center of the divided photo-detector. Therefore, the method of using the divided photo-detector can be applied to a scanning-type laser microscope using a point light source, but it is impossible to simultaneously irradiate the light from the light source to a certain region of the sample, focus the reflected light by an imaging lens, and receive its image by a CCD camera so as to observe in-plane magnetized distribution in an image. For example, even if pixels adjacent to each other of the CCD camera are handled as the divided photo-detector, the light reflected by immediately adjacent spots on the magnetic substance and focused near these pixels is mixed. Accordingly, the method of using the divided photo-detector cannot be applied to a general microscope observing method in which an image is collectively observed by a camera, even if the photo-detector is made in an array form. Furthermore, visual observation through an eyepiece lens is also impossible.
In addition, in the method of using the two-divided or four-divided photo-detector, two orthogonal axes for measuring the in-plane magnetization vector components are fixed to directions determined by divided directions of the photo-detector. Further, a complicated mechanism becomes necessary to make orientation of the photo-detector rotatable in order to freely rotate the axes.